# Computer Network ## TCP/IP 1. 为什么 ack=seq+1 而不是 ack=seq?\ 通过查询资料和所学内容，我们知道： 1. 三次握手的前两次握手，client和server会根据一定的算法(连接四元组和时间戳还有Security Number)生成ISS。 2. 三次握手的后两次握手，client和server分别会对对方发过来的SYN进行应答，在ack序列上等于seq+1。 3. 后面传输数据的时候，sender发送len长度的data，receiver就会将ack(receiver)=seq(sender)+len(sender)。 所以说，3中接收到多少数据就讲多少数据长度的len加在seq上是一个正常的行为，问题出在2的确认SYN中为什么将ack=seq+1，搜索到结果有如下几个： 1. SYN和FIN也是需要确认的信息，所以需要占据一个seq，表示该信息已经被确认，不需要重传。 2. 如果SYN和FIN不占据一个seq，对于FIN而言，四次挥手的第二三次server的挥手除了FLAG外其他其实是一样的，即一样的ack，可能也有一样的seq(没有数据需要传输的情况下)，可能导致client段无法分辨第二次挥手的ACK和第三次挥手的FIN。 Notice that the acknowledgement number has been increased by 1 although no payload data has yet been sent by the client. This is because the presence of the SYN or FIN flag in a received packet triggers an increase of 1 in the sequence. (This does not interfere with the accounting of payload data, because packets with the SYN or FIN flag set do not carry a payload.) 注：收到的报文中只有ACK时不需要将seq+1。 2. TCP传输中ACK的优化\ 已经知道的有延迟ACK，还有就是ACK和需要传输的数据一起过去，SO上面已经有大佬画了图： ``` Before ACK Optimization: +-------------------------------------------------------+ | client network server | +-----------------+ +--------------------| | (connect) | ---- SYN ----> | | | | <-- SYN,ACK -- | (accepted) | | (connected) | ---- ACK ----> | | \_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/ when client sends... \_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/ | | | | | (send) | ---- data ---> | | | | <---- ACK ---- | (data received) | \_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/ when server sends... \_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/ | | | | | | <--- data ---- | (send) | | (data received) | ---- ACK ----> | | \_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/ ...and so on, til the connection is shut down or reset ========================================================= ========================================================= After ACK Optimization: \_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/ | | | | | | <--- data ---- | (send) | | (data received) | | | | (send) | -- data,ACK -> | | | | | (data received) | | | <- data,ACK -- | (send) | | (data received) | | | | (wait a bit) | <--- data ---- | (send) | | (data received) | | | | (send) | -- data,ACK -> | | | | | (data received) | | (send) | ---- data ---> | (wait a bit) | | | | (data received) | | | <- data,ACK -- | (send) | | (data received) | | | | (wait a bit) | (dead air) | | | | ---- ACK ----> | | \_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/ ``` 3. TCP Connection State Diagram in rfc793 ``` +---------+ ---------\ active OPEN | CLOSED | \ ----------- +---------+<---------\ \ create TCB | ^ \ \ snd SYN passive OPEN | | CLOSE \ \ ------------ | | ---------- \ \ create TCB | | delete TCB \ \ V | \ \ +---------+ CLOSE | \ | LISTEN | ---------- | | +---------+ delete TCB | | rcv SYN | | SEND | | ----------- | | ------- | V +---------+ snd SYN,ACK / \ snd SYN +---------+ | |<----------------- ------------------>| | | SYN | rcv SYN | SYN | | RCVD |<-----------------------------------------------| SENT | | | snd ACK | | | |------------------ -------------------| | +---------+ rcv ACK of SYN \ / rcv SYN,ACK +---------+ | -------------- | | ----------- | x | | snd ACK | V V | CLOSE +---------+ | ------- | ESTAB | | snd FIN +---------+ | CLOSE | | rcv FIN V ------- | | ------- +---------+ snd FIN / \ snd ACK +---------+ | FIN |<----------------- ------------------>| CLOSE | | WAIT-1 |------------------ | WAIT | +---------+ rcv FIN \ +---------+ | rcv ACK of FIN ------- | CLOSE | | -------------- snd ACK | ------- | V x V snd FIN V +---------+ +---------+ +---------+ |FINWAIT-2| | CLOSING | | LAST-ACK| +---------+ +---------+ +---------+ | rcv ACK of FIN | rcv ACK of FIN | | rcv FIN -------------- | Timeout=2MSL -------------- | | ------- x V ------------ x V \ snd ACK +---------+delete TCB +---------+ ------------------------>|TIME WAIT|------------------>| CLOSED | +---------+ +---------+ ``` ## Reference 1. [StackOverflow | Does TCP send a SYN/ACK on every packet or only on the first connection?](https://stackoverflow.com/questions/3604485/does-tcp-send-a-syn-ack-on-every-packet-or-only-on-the-first-connection?rq=1\&newreg=b5892dcf2e9e453685c3dd626d1c5082) 2. [Understanding TCP Sequence and Acknowledgment Numbers](https://packetlife.net/blog/2010/jun/7/understanding-tcp-sequence-acknowledgment-numbers/) 3. [Why does an pure ACK increment the sequence number?](https://networkengineering.stackexchange.com/questions/48775/why-does-an-pure-ack-increment-the-sequence-number) 4. [RFC 793](https://tools.ietf.org/html/rfc793) --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://legacy.cookielau.com/archives/12-interview/1-computernetwork.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.